The tire construction concept, radial tires have a complex structure, as seen in FIG. 1, which includes the following key components:
Tread (P1): area where the tire touches the ground; Shoulder (P2): Transition area from the tread to the tire sidewalls; Sidewalls (P3): area that supports the tire structure; Bead (P4): area of contact and fixation of the tire with the wheel for assembly; Casing ply (P5): the steel or textile structure that conforms the tire. In radial tires it is 100% metal or mixed, textile with metal, with differentiated construction of the diagonal tire, which usually has a textile structure and ply angle and different formatting; Work plies (P6): these are the tread plies and their function is to conform, structure and move the tire; Guard plies (P7), and the purpose of these plies is to protect the tread structure; and Inner casing area (P8): area where air or the air tube is housed in tubeless tires.
The vulcanization concept: invented in 1839 by Charles Goodyear, an American inventor, vulcanization generally consists of applying heat and pressure to a rubber composition to give it the shape and properties of the final product. Rubber is heated in the presence of sulfur and accelerating and activating agents to form cross links in the individual polymer molecules, which account for the development of a three-dimensional rigid structure with resistance proportional to the amount of such links. It must be pointed out that there is also a process of cold vulcanization.
The tire retreading concept: this process reutilizes used tires. The key vulcanization areas are determined as shown in FIG. 2: Area (A1): Connecting rubber on the new tread applied. This is the rubber to be vulcanized to link the casing to the new materials applied to the tread area; Area (A2): repair and buffing area where filling rubber will be applied to fill it, and also patches for inner reinforcement; Area (A3): connecting rubber on the patches. This is the rubber to be vulcanized to link the inner structure of the casing to the patch that reinforces damaged areas; Area (A4): Rubber with several compounds subject to vulcanization in several areas of the tire as a means to repair or to actually retread the tire.
The retreading process: vulcanization is applied to add rubber to a “used tire” so it can be reused. In cargo loading, machines and agricultural fields, the casing structure corresponds to a range of up to 82% of the casing structure, meaning that lack of reutilization is a major waste. The “prepared tire” represents the final product prepared to be vulcanized. A new “tread” is applied to this tire (alternatively the tire can be prepared for partial repairs in parts of its structure that got damaged when the tire was running) To retread the tire, the following sequence of steps is followed as defined in the block diagram shown in FIG. 3, describing the steps:
Tire preparation stage (1): also called “envelope”. This is the application of a rubber envelope covering the entire outer area of the tire. The end is sealed by the tire bead or by the wheel or by the sidewall, depending on the length of the wheel flange. If it is long, it will require using an envelope rim. If it is short, it will be shaped as a flange covering part of the sidewalls and requires a shorter length for covering and sealing. In the market it is called a flanged envelope.
The purpose of applying an envelope is to insulate the used tire from the inner atmosphere of the autoclaving equipment. The envelope has a nozzle that provides connection to a hose inside the autoclave.
The types of envelope used: 1) Flanged envelope and bead: they differ basically in flange length. When it is long, it can be used in sealing rim assemblies or else with short-flanged wheels; and 2) Conjugated envelope or “innerlop”: This type does not require assembly with rims or wheels. Sealing is made with a rubber element that coats the inner part of the tire and replaces the air tube. It is differentiated by the fact that the conjugate has only one point of sealing and the innerlop is sealed in both sidewalls of the tire.
The connection of the envelope nozzle to the autoclave inner hose keeps the tire atmosphere under control of the autoclave external pressure. Therefore, when the autoclave internal atmosphere is pressurized, it causes a negative pressure differential thus causing the outer atmosphere to which the tire is connected to always have a smaller pressure than the autoclave inner atmosphere. The main consequence is the fact that, as this envelope is made of an extremely flexible rubber, the pressure differential forces out the air between the envelope and the tire. This causes the envelope to exert compression on the whole periphery of the tire, which makes it possible for the new tread or any other rubber to be vulcanized to get suitable pressure so that the parts unite.
Usually the level of negative pressure contained within the tire envelope can be controlled by injecting air through the hose contacting the atmosphere, and the application of compressed air can control the level of mechanical pressure made by the envelope on the tire.
Tire/wheel assembly stage_(2): used in a system called “double pressure”. In this case, an “air bag” component is used. This is a reinforced air tube designed to deliver resistance at high pressure and temperature. It forces the compressed air inside it to exert pressure on the sides and the tire bead against the wheel, causing sealing of the envelope with the autoclave inner atmosphere. To get this effect, the air bag pressure must be higher at all times to get the differential that will keep envelope sealing on the tire. A protective component is also projected. This is a rubber element assembled between the wheel and the air bag.
The wheel is a metal element assembled between the tire beads designed to keep the air bag inside the tire, compressed by the pressure applied inside it and to work as sealing of the edges of the vulcanization envelope;
Finally, when the tire is assembled on the wheel, a “sealing rim” element is assembled. This is a metal element assembled individually on each of the beads to replace the function exerted by the wheel to seal the outer surface of the tire wrapped by the vulcanization envelope, thus dismissing the need for an air tube and the rubber guard used between the wheel and the tube.
Stage of vulcanization in autoclave_(3): the prepared tire is introduced inside the autoclave where it is exposed to special conditions of pressure, temperature and vulcanization time.
Autoclave unloading stage_(4).
Tire dismounting stage_(5).
Tire cooling stage_(6).
The basic parameters of the autoclave vulcanization stage: to get successful tire vulcanization, the basic variables involved in the process are:
Vulcanization pressure: the pressure exerted on the compounds subjected to vulcanization is a key element to join the two surfaces considering the structure of the material and the conditions of use of the tire which require a strong link between the tire surface and the new rubber to be vulcanized.
Normally, the action of pressure on the rubber compound to be vulcanized in the autoclave is made by the compressed air inside the autoclave which works on the tire via pressure differential exerted by the application of the sealed envelope on the tire which can be assembled with an air bag or by sealing rims.
A1. Pressure parameter in the single pressure system: also known as “disassembled”. We find a pressure differential only between the envelope and the tire. Therefore, what determines the compression of materials is the compressed air inside the autoclave and the pressure differential resulting form the connection of the envelope with the autoclave outer atmosphere. The following is considered as pressurization parameters of the autoclave: 4-6 kg/cm2. The differential pressure between the autoclave inner atmosphere and the inside of the envelope is usually given by the pressure inside the autoclave, which can be reduced during the process via injection of compressed air, to keep the same differential existing between the autoclave and the air bag, considering that equalizing the two pressures would cause loss of compression of the envelope on the tire and would damage adhesion of the two materials.
A2. Pressure parameter in double and triple pressure systems: for pressurization systems where the air bag is subjected to the action of two inflating pressures, the first one due to the need to keep the envelope tightly sealed in the bead edge and to preserve the structural conformation of the tire, this must be the higher pressure of the process. The other is the pressure inside the autoclave which compresses the envelope against the tire in function of the atmosphere between the tire and the envelope. It is linked to the autoclave outer atmosphere causing a negative pressure differential between the inside of the machine and the outer atmosphere connected by the hose which is connected to the ambient pressure. Pressures normally ranging from 4-6 kg/cm2 are considered as autoclave pressure parameters. The recommended pressure differential between the autoclave and the tire air bag is in the 1.5-3.0 kg/cm2 range.
B. Vulcanization temperature parameter: Applying temperature (or a temperature range) correctly is critical to take rubber to a state in which elastic properties are delivered, reset or improved. In the tire retreading field, there is a range of materials used in several parts of a tire and there is also a variety of formulations suitable to different tire applications and uses made by end users. In the area of retreading, vulcanization temperatures in the 100° C.-150° C. were set as vulcanization temperature parameters where practically all the range of commercially used materials are submitted to vulcanization.
C. Vulcanization times in the autoclave vulcanizing process:
C1. Autoclave pre-heating time: comprises the time the equipment takes to go from ambient temperature to reach the pre-determined vulcanization temperature. This does not mean that the tire follows this temperature; it only refers to the temperature reading inside the equipment. This time varies considerably according to installation conditions of the equipment, ambient temperature, equipment model, heat exchange systems, etc. The acceptable time in the retreading process is 15-45 minutes.
C2. Vulcanization time: comprises the time between the end of the pre-heating time with temperature stabilized in the operating limits and the time required for all tire areas to get vulcanized. In the area of cargo load tires, depending on the assembly methods, times vary between 105 to 180 minutes. During this time, these are the vulcanization phases:
1. Preliminary phase, set by the time when vulcanization is not yet taking place. During this time rubber behaves thermoplastically and reduces viscosity due to the combined effect of heating and macromolecular splitting. Depending on the choice of the vulcanization system, the start of vulcanization can be quick or slow. In molding processes, it is necessary to keep the compound flow until all tire cavities are filled and air is allowed to escape. Vulcanization starting too quickly is not recommended in most cases because it interferes with the process safety. It may ultimately sometimes cause vulcanization to start before the mold or tire cavities are completely filled.
2. Sub-curing phase, when the reticulation process takes place. At this time most rubber properties are not yet fully developed.
3. Optimal curing phase, with maximum degree of reticulation. As not all properties of vulcanized products reach their optimal value at this curing level, it is necessary to find a balance between the cure stage and a slight over-cure.
4. Over-cure phase, characterized by prolonged heating (vulcanization) of rubbers beyond the optimal curing time.
The heat generating equipment in the tire vulcanization process:    1. Boiler: This is a metal container designed to generate steam by heating water. Direct steam is normally used in equipment such as autoclaves where steam is directly injected in the vulcanization chamber under pressure and the steam gets in direct contact with the rubber to be vulcanized.
Indirect steam is used in structures with different shapes. The commonest ones are: copper, iron or steel coils that circulate steam and transmit heat to the surface and transfer it to environment directly in the case of autoclaves that heat the mold in which it is cast in its body or forming a wrapping around the mold to be heated.    2. Vulcanization chamber: is placed around the vulcanization mold, where steam circulates and heats it.    3. Electrical resistance system.    4. Autoclave: metal container where pressure and temperature are applied for a predetermined time to vulcanize “prepared tires”. The key components of the autoclave are:            Ventilation system: includes a motor, propellers and air conductors along the autoclave inner wall, designed to make convection of heated air and keep air temperature homogeneous in all vulcanization points.        Autoclave inflating system: the purpose is to keep and control compressed air inside the vulcanization chamber.        Air bag inflating system: system including internal piping and hose used to keep and control compressed air in tires when tires are assembled with air bags.        Envelope inflating system: Internal piping and hose system connected to tire envelopes, designed to keep and control pressure exerted by the envelopes on the tire by the pressure differential between the autoclave pressurized atmosphere and the ambient atmosphere. Via controlled injection of compressed air, it is possible to adjust the strength of the compression exerted on the tire and reduce it.        Vacuum system of envelopes: Piping system joint or parallel to the envelope inflating system, designed to keep or control vacuum pumping in envelopes thus making it possible to remove more air from envelopes. Consequently, the compression force is increased on the tire, or drastic reduction of the compression force is reduced in case of accidental punctures in the envelope.        Heating system: these components are designed to vulcanize tires. Several methods may be used: direct or indirect steam, electricity, thermal fluid.        Process control systems: electric or electronic panels that control all time pressure and temperature variables of the vulcanization process. Some record events to be checked later.        Autoclave pressurizing systems:        
1. Single-pressure system: developed with tire assembly only with envelopes called “innerlop” or conjugated envelopes or assembly with vulcanization rims. This system uses only differential pressure between the autoclave inner atmosphere and the atmospheric pressure of the environment. Its use is limited to steel radial tires because bias ply tires or tires textile plies such as nylon, rayon or polyester require assembly with wheels and application of inner pressure so as not to damage tire shape, structure or conformation with the application of temperature on tire components.
2. Double pressure system: developed through wheel assembly, air bag, protector and wheel, which provides application of pressure to the air bag, thus creating a differential atmosphere between the tire and the autoclave inner atmosphere. The purpose of greater pressure in the air bag is to seal the envelope and also to keep the original structure, shape and conformation of the product in case of tires with textile plies such as nylon, rayon or polyester, which are sensitive to increases in temperature.
3. Third pressure system: similar to the double pressure system. The difference is the fact that there is a compressed air injection control system. The compressed air returns through the envelope piping and this keeps the differential pressure between this and the autoclave atmosphere. The purpose is to promote variation in differential pressure and reduce it to diminish wear caused by the envelope conformation pressure on the tread design, causing thinning of the envelope wall and tear or puncture, deformation and stretching.
Critical analysis of the autoclave tire vulcanization process: although the current process performs its primary function of delivering tire vulcanization, particularly in tire retreading, a detailed study of this process allows us to note a series of restrictive aspects, both from the viewpoint of the process itself and from the viewpoint of the final product obtained.
The level of complexity required by the installation before getting to the actual operation involved in tire vulcanization and in retreading by vulcanization is relatively high, and variable in errors and decisions involved are often higher than the technical capacity of the final user involved, particularly in the retreading sector with unskilled labor. Even in relatively large companies, the levels of success, efficiency and sustainability are quite low. We can see that the cost variable is impacted by several factors, such as:    a) Boiler efficiency, as determined by the following parameters:            a1. Temperature of gases (chimney temperature);        a2. Fuel specifications;        a3. Excess air;        a4. Ambient air temperature;        a5. Losses by radiation and convection;            b) Losses in steam piping;            b1.) Improper layout;        b2.) Wrong sizing of pipes;        b3.) Qualification of materials;        b.4) Insufficient insulation;        b.6) Lack of maintenance in piping;        b.7) Improper operation;            c) Losses associated to tire vulcanization equipment:            c.1) Poorly sized installation;        c.2) Deficient or unsuitable insulation;        c.3) Poor quality accessories such as valves, controllers, etc.;        c.4) Wrong sizing of equipment;        c.5) Lack of maintenance;        c.6) Improper operation.        
Consolidation of the critical analysis of the conventional vulcanization process: an analysis of the aspects that compromise vulcanization effectiveness allows us to state that, among the countless aspects listed, waste of energy is of great relevance.
Energy consumption is always high since in order to transfer heat by means of the aforementioned methods, the heating elements invariably operate at a very high temperature so heat exchanges can take place between the bodies within a desirable time length.
Currently all autoclave heating systems available in the market work with the convection and conduction principle, in which heat exchange between tire components occurs. To do so, transformations and changes in state of the materials involved are required. This causes great losses in efficiency because of the machines, labor, process, materials, the environment, etc. Convection heat exchange is really an expensive process due to the difficulties to control energy waste.
From the viewpoint of the tire physical structure, using vulcanization by convection and conduction is extremely slow. The pre-molded material, duly applied on the tread, undergoes slower vulcanization, since vulcanization takes place by irradiation of the outer surface of the tire and of the pre-molded, the thickness of which varies between 12 and 25 mm, thus forcing heat to penetrate this poor heat-conducting material. This slows down vulcanization and causes great inaccuracy. This traditional concept requires long heating time, i.e., above 180 minutes (3 hours) and, as a consequence, it causes low productivity in retreading.